1. Field of the Invention
This invention relates to a motor, and an electric power steering system in which a motor is provided for assisting a steering operation such that the motor is driven and controlled based on a steering torque detected by a torque sensor.
2. Related Art
An electric power steering system for reducing load of a driver is provided with an input shaft connected to a steering member (steering wheel, steering handle etc.), an output shaft connected to control wheels through a pinion and a rack or the like, a connecting shaft that connects the input shaft and the output shaft. Based on a torsion angle caused in the connecting shaft, a steering torque applied to the input shaft is detected by a torque sensor. Then, based on the detected steering torque value, the motor for assisting the steering operation coupled to the output shaft is driven and controlled.
In such an electric power steering system, brushless motors have come to be applied to the motor for assisting the steering operation in recent years. The brushless motor is a motor that is provided with permanent magnets in a rotor and rotates the rotor by controlling a wave form generating circuit which generates magnetic field of rotation in a stator based on a (rotational) position of the rotor. Since no brush is provided, loss or mechanical noise by the brush and electrical noise by the brush are not caused.
FIG. 9 is a longitudinal cross-sectional view showing construction of a conventional motor. In this brushless motor, an output shaft side bearing 85 is supported by an inner circumferential surface of a through hole formed at a central portion of a bracket 81 that closes an opening of a cylindrical motor case 80 having a bottom. Further, the opposite side bearing 86 is supported by an inner circumferential surface of a through hole formed at a central portion of the bottom of the motor case 80.
A stator core 82 is supported by being press-fitted onto an inner circumferential surface of the motor case 80. A rotor 83 is provided around a shaft 84 (rotation shaft) supported by the bearings 85,86 at a position opposing the stator core 82.
The motor case 80 and the bracket 81 are fitted to each other with sliding fitting walls formed in both members.
In Utility model Publication JP-Y2-06-49083 Publication, there is disclosed a stator fixing device in an electric generator in which sliding fitting walls are formed such that an outer circumferential surface of the stator at one side end is press-fitted to an inside of a rear housing of the electric generator.
Conventionally, a brushless motor for assisting a steering operation in an electric power steering system is often configured so that the stator core (or armature) is press-fitted to the motor case as described above. Since the motor case is made of steel, the density of magnetic flux in the core can be changed on its contact surfaces after press-fitting. Accordingly, there is a problem that torque loss is increased. Further, as shown in FIG. 9, it is necessary to provide a space to place the sliding fitting walls for attachment when the motor case is attached to the bracket, a housing of a speed reducer, or the like. Furthermore, due to dimensional tolerances of three parts: the stator core, the motor case and the bracket, center deviation to the rotor can be caused. Therefore, there is a problem that air gap between the rotor and the stator has to be taken large.